U.S. Pat. No. 4,161,356 to J. W. Giffin, M. A. De Santis and J. S. Burchard discloses a prior art housing which has been used for chemical etching of integrated circuit wafers.
U.S. Pat. No. 4,208,240 to T. S. Latos shows a prior art etching end point detector. In this apparatus, coherent light is directed onto the surface being etched, so that changes in reflectivity of the surface upon exposure of the underlying substrate produce a detectable change in the characteristics of the reflected light. The present invention relies upon this same technique, with some important differences which are explained below. In prior art devices, it is typical that an intense light source is used to illuminate a wafer or the like from outside of the housing where etching is done. Specular reflection from the wafer is observed by a detector, usually outside the housing. The fact that both the source and the detector are outside the housing usually requires intense sources and sensitive detectors.
A first level of reflection is observed at the beginning of etching and a second level of reflection, clearly distinct from the first level, is observed with respect to the end point. After the second level is observed for a predetermined time interval, the end point is identified.
The analog electrical signal representing the amplitude of reflectance is differentiated and circuits are provided to detect the transition from one slope condition to another distinguishable slope condition.
A similar approach is taken in U.S. Pat. No. 4,198,261 to H. Busta, R. Lajos and K. Bhasin where sharply different values of light intensity are detected when a desired layer is etched and the next layer reflects a light beam at an intensity different from the former layer.
U.S. Pat. No. 4,068,016 to Wilmanns describes a reflective system for controlling build-up of thin layers of the type described above. In the aforementioned patent, the time derivative signal is compared with a reference signal and the difference is used to regulate the evaporation rate for building up thin layers.
In summary, prior art etching and evaporation control systems of the reflective type rely on the derivative of a signal representing reflectance to attain a measurably different condition relative to a prior condition. One drawback of this approach is that a finite time must pass before the different condition is recognized. In other words, in prior art systems there is a time delay which arises after a characteristic end point condition arises due to the need to verify that the condition has indeed occurred. For example, in U.S. Pat. No. 4,208,240 the end point is recognized after a zero derivative is sensed for a time interval.
An object of the present invention was to devise a reflective end point measurement method which does not require waiting a specified time for ascertaining whether the end point has been reached. A problem of waiting to verify an end point condition is that further etching is occurring which could damage underlying layers.